This application is a 371 of PCT/NL99/00533 dated Aug. 27, 1999.
This invention relates to the application of a coating to various substrates, including the application of barrier layers to plastics.
The provision of coatings on all kinds of substrates, as on plastic film or on articles formed from plastic, on metals, or on other materials, is an important tool for improving the (surface) properties of such substrates. To be considered here are, for instance, improving the wear and/or scratch resistance of plastics, the corrosion resistance of metals, and especially improving the barrier properties of plastics, as in polyester bottles or polyester or polyolefin films. Particularly the last-mentioned application is of great commercial and technical importance, since the greater part of plastics have moderate to poor barrier properties in respect of oxygen and CO2. This means that such plastics are less suitable for use as packing material for oxygen-and CO2-sensitive materials and foods.
Plastic materials that have sufficient barrier properties are, for instance, PVDC, and laminates of non-barrier plastics and other plastics, such as EVOH and the like, or metals, such as aluminum, which have a good barrier function. The use of this kind of systems, however, meets with other objections, for instance from an environmental point of view. On the one hand, chlorine containing plastics are less desirable specifically in garbage incineration, while, on the other hand, multilayer systems possess disadvantages from the viewpoint of reuse.
Accordingly, there is a need for a system enabling a coating to be applied to a plastic or any other substrate, which coating imparts to the substrate additional useful properties. The present invention now provides for this need.
The invention relates, in a first variant, to a method for applying a coating to at least one surface of a substrate, wherein the substrate is introduced into an atmosphere which contains at least one chemically reactive compound and water, and subsequently, using a corona discharge, on the surface of the substrate a coating is formed from the reactive compound.
Surprisingly, it has been found to be possible to form a coating by activating a suitable chemically reactive compound using a corona discharge, in the presence of water, to form a coating on the surface with good properties. Suitable reactive compounds include inter alia silicon-containing compounds which can be brought into the gas phase, such as silanes and siloxanes.
It has been found that for obtaining good properties in respect of the permeability to vapor and/or gas, the presence of water is essential. In case an anhydrous gas phase is used in the corona treatment, the coating obtained is not sufficiently vapor/gas inhibiting. As soon as water is present in the gas phase, however, the permeability of the coating is found to be greatly reduced.
Preferably, the amount of water is more than 0.1% by volume, based on the volume of the gas phase, while the balance consists of inert gas and the chemically reactive compound. More preferably, the amount of water is at least 0.5% by volume, the upper limit being formed by the saturation vapor pressure. Obviously, it is dependent on the temperature, but will not exceed 5% by volume.
Suitable as chemically reactive compound are inter alia the compounds given in Table 1.
The choice of the compound depends on various factors. Primarily, the desired properties of the coating are of importance. What substrate the coating is to be applied to also plays a role.
Suitable substrates are especially metals and plastics. As has already been indicated, the invention can be advantageously used for imparting barrier properties to plastics that do not inherently possess these properties. Examples include polyester bottles and polypropylene films. These materials are often used for packing beverages and foods. It is then of importance that no oxygen can reach them, or that no CO2 can escape. By applying a coating in the manner described hereinabove, a single-layer plastic having a coating thereon that does not give any adverse effects upon subsequent processing and/or reuse is obtained in a simple manner.